The bog at Etang de la Gruère (Jura Mountains, Switzerland) consists of 420 cm of Sphagnum-dominated bog peat overlying 230 cm of Carex-dominated fen peat. One hundred cm below the bog surface, there is a pronounced peak in lead (Pb) concentration (approx. 10 µg/g) which has been dated at 2110 ± 30 BP and can be attributed to Roman Pb mining and smelting.
Lead concentrations in peats from deeper, much older layers were measured using ICP-MS and found to be low and relatively constant (0.28 ± 0.04 µg/g, n = 17) from 405 cm to 235 cm which corresponds to the period from approx. 8,000 and 5,500 years before present (BP). In this same interval, scandium (Sc) concentrations (measured using INAA) were 0.07 ± 0.02 µg/g, yielding an average Pb/Sc ratio of 4.1 ± 1.2. These values are assumed to represent the true "background" Pb and Sc concentrations and Pb/Sc ratios of pre-anthropogenic aerosols. At 205 cm the Pb concentrations began to increase by 2 to 3 times, but these are proportional to the increases in aluminum (Al), titanium (Ti), silicon (Si), and Sc, and reflect an increase in Pb deposition supplied by silicate-derived soil dust. This depth, dated at 5,230 BP, coincides with the development of agriculture and indicates the impact of soil cultivation on metal fluxes to the air. At 115 cm, however, the Pb concentrations increase out of proportion with Sc; this point was dated at 3,000 BP and reflects the beginning of Pb contamination by mining and metallurgy in Europe and the Middle East.
There are two pronounced peaks in Pb concentrations centered at 435 cm and 555 cm, corresponding with local maxima in ash and ash-forming major elements at the same depths. These samples have been dated at 8,230 BP and 10,590 BP, respectively, indicating the Vasset/Killian volcanic events (Massif Central, France) and Younger-Dryas cold phase as the most likely explanations.